This patch adds a missing docstrings for exported identifiers, and
adjust some of the existing ones to match the standard style.
In some cases, the identifiers were un-exported after noticing they had
no external users.
Besides improving documentation, it also reduces the linter noise
significantly.
We will need a few string sets in different places, create it as a separate
package for convenience.
For now, it only implements strings, but it may be extended to other data
types in the future.
